1. Field of the Invention
The invention relates to a process and apparatus for producing boards made of derived timber products or laminated veneer boards.
2. Description of the Related Art
DE 197 18 772.2 (corresponding to U.S. patent application Ser. No. 09/071,778, filed May 4, 1998, which is hereby incorporated by reference) discloses a process by means of which disadvantages and problems associated with conventional preheating of wood products by means of high-frequency or microwave energy, such as an excessive duration of action, over-long energy-intensive preheating sections and penetration with the use of high-frequency or microwave energy, are resolved by adjusting the moisture content of the material to be compressed before it enters the press. The moisture content is adjusted in such a way that, while ensuring a significant reduction in the pressing factor, it is possible to ensure adequate transverse tensile strength within the pressing section or during the pressing time by increasing the preheating temperature of the material to be compressed to over 80xc2x0 Celsius. The feeding and removal of the material to be compressed into the microwave device or from it, respectively, should be configured in such an optimum mannerxe2x80x94the material to be compressed being preheated to over 80xc2x0 Celsius within a short timexe2x80x94that the chemical reaction for curing the binder starts in the region of the press or after the start of compression.
This result is achieved, as regards the disclosure in DE 197 18 772.2, by the fact that the preheating of the core of the material to be compressed to greater than or equal to 85xc2x0 Celsius takes place after or during the precompaction process by means of traveling-wave microwave energy and its reflection in an interaction, at the center of the material to be compressed, between the energy emitted and the energy reflected, focusing of the radiant energy in the central cross section being effected with a large energy-input and energy-absorption angle xcex2 for an increased heating gradient, and the preheated mat of material to be pressed entering the pressing area of the press with a moisture content which is 15% to 30% lower than the conventional standard moisture content.
This solution provides the following advantages. Due to the effect of increased microwave energy density in the center of the material to be compressed, with the microwave energy that is not absorbed by the loose material being directed back into the center of the mat of material to be compressed by additional microwave reflector surfaces and control devices in a process involving the interaction of the energy emitted and the energy reflected and the focusing of the wave energy in the core of the mat, the preheating temperature in the core of the material to be compressed is achieved over a considerably shorter distance, approximately in a ratio of about 10:1 to the prior art. Since the heating gradient or energy input and energy absorption angle xcex2 is significantly larger when the concentrated energy is introduced into the center of the board than the angle a with the conventional technique, a higher temperature level can be introduced in the core of the loosely deposited mat of material to be compressed using current testing techniques, more specifically, about 85xc2x0 Celsius. Although the binder may start to cure prematurely within the short time it takes to reach 85xc2x0 Celsius, the reaction time remaining until the mat enters the continuously operating press or until the beginning of the pressing cycle in a single- or multi-opening press is negligible.
The focusing of the traveling waves in the center of the mat of material to be pressed furthermore ensures that 85xc2x0 Celsius in the core of the mat is reached within a very short time. Because of the focusing of the radiant energy in the center of the mat of material to be compressed, the cover-layer area receives less heat. However, when the material to be compressed is introduced into the press the heat energy still required at the outer edges is rapidly made up by the input of heat energy from outside, for example, from the heated steel belts, resulting ultimately in a further shortening of the pressing factor by about 50%, depending on the thickness of the material to be compressed. This is achieved by displacing the 100xc2x0-Celsius steam point in the center of the material to be compressed toward the start of the pressing section. Without preheating, this 100xc2x0 Celsius steam point is at about 75% to 85% of the total pressed length. Because of the increased energy density, this 100xc2x0-Celsius steam point is displaced into the forward area of the pressing section, corresponding to about 35% to 50% of the pressing length or pressing time.
A process for producing laminated veneer boards has been disclosed in DE-A 196 27 024. This process is based on the object of combining laminated assemblies of large area veneer panels (or sheets) automatically and continuously with adequate mechanical connection so that, given the subsequent continuous pressing operation, the veneer-panel interlinking points have virtually the same physical strength properties in the finished laminated veneer boards as the laminated veneer boards produced according to the previous laminated veneer assemblies of sandwich construction. Thus, it is possible to produce laminated veneer boards of good quality if all the manufacturing or production parameters are matched to one another in an optimum manner and the apparatus operates accordingly. Problems arise, however, from the fact that the moisture content of the layers of glue on the surfaces of the veneer panels and the moisture content of the veneer panels themselves are either too low or too high. If the moisture content is too high, there is the risk that an air/vapor mixture will lead to the formation of vapor in capillaries, resulting in splits in the finished product. If the moisture content is too low, the strength properties of the laminated veneer boards are inadequate. With regards to process engineering, heat transfer to the center, as in the production of particle board and fiber board, is not possible in the course of vapor generation during the production of plywood or LVL boards because the veneer panels form a natural vapor barrier.
The object of the present invention is to indicate a process and an apparatus by which higher quality and, through a more rapid sequence, a higher throughput rate can be achieved in the production of laminated veneer boards, and by which the above-noted advantages of DE 197 18 772.2 can be exploited in the production of laminated veneer boards.
According to a first embodiment of the invention, a method of producing endless laminated veneer boards comprises the steps of: applying an adhesive to at least some of a plurality of veneer panels; assembling the plurality of veneer panels in a plurality of interlinking layers to form one or more cover-layer strands, each of the cover-layer strands having central panels and outer panels; combining one or more of the plurality of cover-layer strands to form a veneer-panel strand, the veneer-panel strand having outer layers and central layers; and preheating the veneer-panel strand at a central region of the veneer-panel strand, wherein the step of preheating further comprises focusing of energy and a reflection of the energy in the central region; and compressing the veneer-panel strand in a pressing area.
The step of preheating may include preheating the central region to greater than or equal to 85 degrees Celsius.
The step of focusing may effect a large energy-absorption angle.
The moisture content of at least one of the adhesive and the veneer panels is preferably greater at the central layers than at the outer layers
The moisture content of at least one of the adhesive and the veneer panels is preferably about 7.8% for the central layers and about 6% for the outer layers.
During the step of assembling, direction of fibers in one of the plurality of layers of the veneer panels may be placed at a 90-degree offset to an adjacent layer.
An apparatus according to the present invention comprises: at least one adhesive applicator for applying adhesive to veneer panels; at least one veneer-panel laying device for interlinking layers of the veneer panels to form a cover-layer strand; a preliminary press comprising a high-frequency or microwave-energy preheating device, the preheating device being adapted to focus energy and a reflection of the energy in a central region of a veneer-panel strand; wherein a moisture content of at least one of the adhesive and the veneer panels is greater at the central layers than at the outer layers.